This invention relates to a screen for reflecting a projected image, in particular to an active screen for reflecting a projected image having improved contrast, and to a projection system incorporating such a screen.
When an image is to be viewed by a group of people, it is common to magnify the image by projecting it with a projector onto a screen having a diffusely reflective surface. Screens for indoor use typically have an area of a few square meters, and screens for outdoor use may have an area of 10 m2 or more.
However, ambient light reflected by the screen can cause the image to suffer from poor contrast, requiring a more powerful projector to be used. Although the amount of ambient light can be reduced by viewing the image in a darkened room, this is not always possible and can be inconvenient.
It is known to limit the amount of ambient light reflected towards a viewer by using a reflective material whose reflectance decreases at angles of incidence below about 45 degrees. However, this may result in an undesirable reduction in the range of satisfactory viewing angles. Also, some ambient light will come from the same direction as the projected light, limiting the effectiveness of this approach.
To achieve a high contrast, plasma addressed liquid crystal displays and field emission displays have been used, but these can be expensive when used to form a display whose area is of the order of 1 m2 or above.
According to one aspect of the invention, there is provided a projection screen for reflecting from a first side a projected image with enhanced contrast, comprising: an active layer whose reflectance can be changed under the influence of an electric field; at least a first electrode for applying an electric field across the active layer; and a photosensitive material whose electrical or dielectric properties can be changed under illumination, wherein the photosensitive material is arranged such that the electrical field across local regions of the active layer is dependent on the illumination incident upon the photosensitive material in each such local region with the result that reflectance of the active layer is locally dependent upon an intensity of light incident upon the first side of the projection screen in that local region.
Preferably, the photosensitive material is a photoconductor adapted to control current through the first electrode. In another advantageous arrangement, there is also a second electrode, whereby the electric field across the active region is applied between the first electrode and the second electrode. In this case, the photosensitive material may be a photoconductor adapted to control current through either electrode.
One of the electrodes may be situated between the first side of the projection screen and the photosensitive material, in which case this electrode is preferably transparent. It will be understood that the term transparent includes the possibility that some absorption will occur.
Because the optical properties of the screen can be changed in localised areas without changing the optical properties of the whole screen, an image with improved contrast and sharpness can be obtained.
Since the screen has a layer structure, it can conveniently be made flat over a large area and mounted vertically, for example on a wall, so that it can be viewed by a group of people.
Since the projection screen does not generate light, it will have a lower power consumption than emissive displays such as field emission displays or cathode ray tubes, and will be more suitable for displaying an image over a large area.
The local reflectance of the screen may be a function of the light incident upon it, in which case an image will be projected onto the front of the screen so that a reflected image can be viewed. In one embodiment, the local reflectance of the screen at a point will increase when the intensity of light incident on the screen at that point is increased. This will cause the screen to reflect more light in the bright areas of a projected image, thereby improving the contrast of the reflected image relative to the contrast which would be obtained with a screen having a uniform reflectance.
An active layer will preferably be used whose reflectance can be changed under the influence of an electric field, such that the active layer can be in a reflective state in which it reflects light, or in a less reflective state in which it reflects less or no light. In those regions of screen where the active layer is in the reflective state, visible light incident on the front of the screen will be reflected, so as to form an image that can be viewed. The active layer may be a normal mode active layer which is in a non reflective state when an electric field above a threshold value is applied across it and in a reflective state when the electric field applied across it is below the threshold value. However, a reverse mode active layer may be used which is in a reflective state only when an electric field above a threshold value is applied.
It will be understood that the bulk and/or interface optical properties of the active layer may be responsible for the changes in reflectance. Preferably, the reflectance of the active layer will be due to light scattering within the bulk of the active layer, and scattering will be reduced when the active layer is in the non reflective state. The active layer may comprise a liquid crystal material. Preferably, the active layer will comprise regions of liquid crystal material or other optically active material embedded in a structural matrix. In a preferred embodiment, the active layer will be a Polymer Dispersed Liquid Crystal material, comprising liquid crystal micro droplets or pockets embedded in a polymer matrix.
Preferably, the active layer will be an electrically insulating layer so that an electric field can be applied across it when desired. However, in one embodiment, the active layer comprises a transparent photoconductor material.
The threshold electric field value at which the active layer makes a transition from a reflective state to a non reflective state will preferably be well defined, in order to selectively switch off the reflectance of the screen in regions where the incident light is below a pre-set value. However, it will be appreciated that the transition will in practice occur within a range of electric field values if for example the microdroplets are not of a uniform size, or there are other inhomogeneities in the active layer.
Preferably, the pre-set level of incident light intensity at which the active layer changes state will be chosen such that it is above the ambient light level. In the dark areas of the projected image where only ambient light is incident on the screen, the screen will remain dark, thereby improving the contrast of the image. Preferably, a potentiometer or other adjusting means will be provided to adjust the threshold light level at which the active layer changes state, so that the screen can provide good contrast under a range of ambient light conditions. The adjusting means may be manual, or alternatively automatic adjusting means may be provided, comprising an electronic circuit having a light sensor, for example.
The active layer may be transparent when it is in the non reflective state if the screen is used to display a reflected image, in which case an absorbing surface will preferably be located behind the active layer. This will cause the screen to have a dark appearance in the dark areas of the image incident on the screen.
The photosensitive material will preferably be a photoconductor that only conducts under illumination within a predetermined frequency range, at typical operating temperatures such as room temperature. However, a material with a light dependent dielectric constant may be used in the case where current is not required to pass through the photosensitive material.
The photosensitive material may be in the form of a layer situated adjacent to the active layer, between the first and second electrodes. If the photosensitive layer is opaque, then it will preferably be situated behind the active layer, but if the photosensitive layer is transparent, it may be situated in front of the active layer.
In one embodiment, a reverse mode active layer and a layer of photoconductor are situated adjacent to one another, between the first and second electrodes. At the points where light of sufficient intensity is incident on the screen, the photoconductor will conduct, thereby increasing the electric field across the active layer and the reflectance of the active layer at those points. In the regions where the photoconductor remains insulating, the electric field across the active layer will not change significantly. Since in this embodiment the active layer is an insulator, no significant current flows through the photoconductor, and little power is consumed.
In another embodiment, the photoconductor will form part of the active layer, such that the active layer comprises regions of liquid crystal material or other optically active material embedded in a structural matrix, wherein each region of liquid crystal is bounded at least in part by a layer of transparent photoconductor.
In another embodiment, the active layer will comprise a plurality of separated regions of optically active material embedded in a structural matrix of transparent photoconductor material. The active layer will preferably be contacted on each side by a first electrode and second electrodes respectively so that an electric field can be applied across the active layer in the regions where the photoconductor is in the insulating state.
At least the first electrode may be structured with high and low resistance regions such that a local electric field can be applied across the active layer when a current is passed through the first electrode. This will allow adjacent localised areas of the screen having different levels of illumination to have a different reflectance, thereby preserving the sharpness of the reflected image. The first electrode may be patterned as a unitary part, or the low resistance regions may be deposited on a high resistance sheet. Alternatively, the low resistance regions may be formed from a low resistance matrix. The high resistance regions may be formed by resistive links in electrical contact with the photoconductor. The first electrode will thereby provide low resistance delivery of current to a plurality of high resistance regions.
So that light can be reflected by the active layer without significant attenuation, the photoconductor will preferably be located behind the active layer in the direction of incident light. In order to allow sufficient light to reach the photoconductor when the active layer is in the reflecting state, a plurality of windows may be provided through the active layer. The windows may have side walls that are aligned perpendicular to the surface of the active layer, or alternatively the side walls may be inclined in order to control the solid angle within which light can pass through a window and reach the photoconductor.
An optical element such as a lens may be provided in each window to further control the solid angle within which light can pass through a window and reach the photoconductor.
Alternatively, the photoconducting material may extend though the windows in the active layer and make electrical contact with the first electrode and the second electrode, which will be situated on either side of the active layer. When light is incident on the photoconductor, a conducting path will be formed between the two electrodes, which will reduce the electric field across the active layer in the region where the conducting path has been formed, and increase the reflectance of the screen in that region.
The projection screen may comprise at least a third electrode, the screen being arranged such that the active layer lies between the first electrode and the second electrode, and the photoconductor is between the first electrode and the third electrode. Current passing through a region of photoconductor will pass through the first and third electrodes, which will alter the potential in one or more regions of the first electrode and will consequently change the electric field across the active layer in those regions.
The second electrode will preferably be a low resistance electrode, and if a third electrode is used, this will also be a low resistance electrode, so that each of these electrodes remain close to an equipotential in order to reduce the risk that the contrast of the reflected image will be distorted, particularly if a current is passed through an electrode. One or more of the electrodes will be preferably planar, to make the electric field across the active layer more controllable.
In some situations such as outdoors, the ambient light level may be similar to or higher than the light levels of the visible projected image. Therefore, a control image in the near infra red or other invisible wavelength range may be added to the projected light forming the visible image on the screen, the control image being representative of the intensity distribution of the visible image.
The screen will preferably be adapted such that the optical properties of the screen are only affected by the control image. In one embodiment, the screen will be adapted such that the optical properties of the screen only change in response to light incident on the screen within a pre determined range of solid angle, and angle of incidence of the visible image and the control image will be different, such that the optical properties of the screen only change in response to the control image.
Alternatively, or in addition, a photosensitive material that is not significantly sensitive to visible radiation may be used so that the reflectance or other optical properties of the screen are controlled by the control image. This will allow the screen to be in a non reflective state in the dark areas of the image, even if the visible ambient light level is high.
If a photoconductor is used that is sensitive to visible radiation, a filter may be provided to shield the photoconductor from radiation in the visible range. Alternatively, the photoconductor may be shaped so that its volume forms a resonant cavity tuned to the desired wavelength.
The control image may be inverted with respect to the visible image, such that maxima in intensity in the visible image correspond to minima in the infra red image. If such an inverted image is formed, a screen will be used where the reflectance decreases with increasing light intensity.
It will be understood that the intensity of the control image will not necessarily be proportional to intensity of the visible image at each point. The functional relationship between the intensity of the control image and that of the visible image may be non linear, to compensate for the fact that the reflectance of the screen in the visible range may not be proportional to the intensity of invisible light incident upon it.
Since a screen will be used as part of a projecting system, according to a yet further aspect of the invention, there is provided a projection system, comprising a projection screen as described above and a projector for projecting an image on the projection screen.
The projector may be adapted to project an invisible control image onto the screen and the screen may be adapted so that the reflectance at different points on the screen is determined at least in part by the light intensity of the control image at those points. The control image can then be used to increase the contrast of a visible image projected coincidentally with the control image. Alternatively, the projector may only project a control image, and the control image may be used to modulate the reflectance of ambient light on the screen, so that ambient light reflected from the screen forms an image representative of the control image.
The control image and the projected image may be projected onto the screen by separate projectors, in which case the projection system will comprise a main projector for projecting a visible image and a control projector for projecting the control image. Advantageously to achieve most effective contrast, the invisible control image may be provided at a higher resolution than the visible image.
A projector may project the control image onto the front of the screen, in which case the reflectance of the screen will be controlled by the control image. Alternatively, a control image projected onto the back of the screen may be used to control the reflectance on the front side of the screen, so that a visible reflected image may be formed thereon.
The projection system may be used to produce a colour image. In one embodiment, to produce an image that is in colour, the projector will be adapted to project a sequence of monochrome images, each monochrome image having a different single colour, wherein the monochrome images are projected one after another sufficiently rapidly that they are perceived as a single image having a plurality of colours. Each monochrome image will preferably be produced by projecting a control image simultaneously or immediately before a single colour is projected onto the screen.
Where the control image is invisible, visible light which is uniform or which carries a different pattern to the control image may be projected onto the screen to form a visible display corresponding to a representation of the control image. Since the image is formed by changing the optical properties of the screen rather than by electroluminescence or photoluminescence, this yet further aspect of the invention provides a simple way of viewing an invisible image.
Preferably, the reflectance or the transmission of the screen will be controlled by the control image, so that at each point on the screen, visible light incident on the screen remains at the same frequency when it is reflected or transmitted by the screen.
The source of invisible radiation may be a projector, and the control image projected onto the screen may control the screen""s reflectance, so that light such as visible ambient light falling uniformly on the screen is non uniformly reflected by the screen and forms of an image thereon which can be viewed. Alternatively, a white light source may be placed behind the screen, and the control image may be used to control the transmission of the screen at different points.
The source of invisible radiation may occur naturally, and may be formed by hot objects emitting black body radiation, such as burning wood in a fire. The display screen may be used as a visor on a fireman""s helmet, so that he is better able to see hot objects.
In another embodiment, the display system is used as an infra red night vision system, and visible light incident on the screen is optionally provided by an electrically powered light source.